1. Field of the Invention
This invention generally relates to a motor control system particularly suitable for applications wherein the motor must be very accurately controlled as in the case of an apparatus for separating cellular components, such as red blood cells and platelets, from blood plasma by centrifugation. More specifically, the invention relates to a control circuit for a centrifuge for improving the degree of separation of the cellular components from the plasma.
2. Description of the Prior Art
Various motor control circuits usable with centrifugal blood separators are known. Generally, such motor control circuits control the separation apparatus to separate cellular components, such as red blood cells and platelets, from plasma. Once the plasma is separated, various tests can be performed to detect, for example, triglycerides, potassium or cholesterol in the blood. The accuracy of such tests is a function of the degree of separation of the cellular components from the plasma which depends on a variety of factors. The most important factors are the rotational speed of the centrifuge and the spin time. More specifically, the degree of separation of the cellular components from the plasma is primarily dependent upon the product of the rotational speed of the centrifuge and the spin time or time of operation of the centrifuge. Blood separators which are operated at relatively low speed time products do not result in adequate separation (e.g., red blood cells and platelets remain in the plasma). When blood separators are operated at relatively high speed time products, the degree of separation is relatively unaffected although problems have resulted.
A sample cup is located within the centrifuge and is adapted to receive the whole blood to be separated. The sample cup is carried by a holder which is coupled to the shaft of the centrifuge drive motor and thus is rotated at the same speed as the motor shaft. When the centrifuge is operated at a relatively high speed the sample cup may rupture due to internal pressure and consequently leak.
Other factors also affect the degree of separation of the cellular component from the plasma. For example, the drive motor acceleration can affect the degree of separation. Specifically, when the drive motor is accelerated too rapidly, several problems can result such as, air bubbles being trapped and breakage of red blood cells. Broken red blood cells can adversely affect some tests, such as the potassium test. Also rapid deceleration of the drive motor can result in remixing of the plasma and the red blood cells in the sample cup.
Moreover, certain factors which may not affect the degree of separation can have an adverse impact on the blood separator itself. For example, operating the blood separator at a relatively high speed does not significantly affect the degree of separation but does affect the life of the drive motor. Specifically the motor bearing and brush wear are dependent upon the speed of the motor and the load. Consequently, operating the blood separator at a relatively high speed will shorten the effective motor life. On the other hand, operation of the blood separator at too low of speed, irrespective of the spin time, can result in inadequate separation.
Various attempts have been made to control the plasma quality by controlling the speed of the centrifuge drive motor and the spin time. For example, in one blood separator apparatus, an interval timer was used to control the spin time by permitting the motor to be energized for a predetermined amount of time each time electrical power was applied to the motor. However, due to the poor regulation of the source of electric power for the motor, the speed of the motor varied substantially. As such, the variation in motor speed produced unacceptable and unpredictable results.
Another attempt to control the speed time product of a blood centrifuge included a servo system having a motor speed detector connected in a feedback circuit. In that system the actual motor speed is detected by a speed transducer and compared with a speed command signal. The difference between the actual speed and the command speed is used to generate an error signal which, in turn, is used to increase or decrease the motor speed to reduce the error. Although such a system adequately controls the voltage and consequently the speed of the centrifuge drive motor, it requires the use of a motor speed transducer, such as a tachometer, which can be quite expensive. However, it is known by those of ordinary skill in the art that the speed of a DC motor can also be determined by measuring the back EMF of the motor. Since the back EMF is directly proportional to the speed of the motor a speed transducer can be eliminated. However, unless the source of electrical power to the motor is regulated, variations in the voltage can cause the back EMF detection circuits to vary substantially which will in turn affect the motor speed.
Another alternative is to utilize a regulated voltage supply for the motor. However, as will be discussed below in connection with the description of FIG. 2, known regulated voltage regulator circuits can be temperature dependent. Specifically, some known voltage regulator circuits require the use of external capacitors. It is well known in the art that the charge on a capacitor operated with a direct current voltage can leak. It is also known that the leakage current increases approximately exponentially with increasing temperature. As will be discussed in connection with the description of FIG. 2, the capacitor leakage current can cause errors in the regulated output voltage. Consequently, since the speed of the motor is proportional to the voltage applied to its terminal, such variations, which are temperature dependent, can cause variations in the motor speed and consequently affect the degree of separation of the cellular components from the plasma.